A Brief History of Mind-Bending Ideas About Black Holes

An artist conception of a black hole spewing radiation all around it. Image: NASA

Physicist Stephen Hawking made headlines recently by saying that black holes – the incredibly massive astronomical objects that made him famous – do not exist. Or they exist, but not how we think. Or something. The truth is complicated.

In fact, to really understand where Hawking and the rest of the astrophysics community are coming from, it’s important to know a little history. Just how we arrived at this complex situation is strange, involving a spate of discoveries about the properties of black holes, each solving some previous problem. But, like a hydra sprouting new heads for each one cut off, the solutions generated new difficulties, eventually leading to Hawking’s recent declaration.

The story starts in 1784, when a geologist named John Michell was thinking deeply about Isaac Newton’s theory of gravity. In Newtonian physics, a cannonball can be shot into orbit around the Earth if it surpasses a particular speed, known as the planet’s escape velocity. This speed depends on the mass and radius of the object you are trying to escape from. Michell’s insight was to imagine a body whose escape velocity was so great that it exceeded the speed of light – 300,000 kilometers per second – first measured in 1676 by the Danish astronomer Ole Romer.

Michell presented his results to other scientists, who speculated that massive “dark stars” might exist in abundance in the sky but be invisible because light can’t escape their surfaces. The French mathematician Pierre-Simon Laplace later made an independent discovery of these “dark stars” and both luminaries correctly calculated the very small radius – 6 kilometers – such an object would have if it were as massive as our sun.

After the revolutions of 20th century physics, black holes got much weirder. In 1916, a short while after Einstein published the complex equations underpinning General Relativity (which Einstein himself couldn’t entirely solve), a German astronomer named Karl Schwarzschild showed that a massive object squeezed to a single point would warp space around it so much that even light couldn’t escape. Though the cartoon version of black holes has them sucking everything up like a vacuum cleaner, light would only be unable to escape Schwarzschild’s object if it was inside a particular radius, called the Schwarzschild radius. Beyond this “event horizon,” you could safely leave the vicinity of a black hole.

Neither Schwarzschild nor Einstein believed this object was anything other than a mathematical curiosity. It took a much better understanding of the lives of stars before black holes were taken seriously. You see, a star only works because it preserves a delicate balance between gravity, which is constantly trying to pull its mass inward, and the nuclear furnace in its belly, which exerts pressure outward. At some point a star runs out of fuel and the fusion at its core turns off. Gravity is given the upper hand, causing the star to collapse. For stars like our sun, this collapse is halted when the electrons in the star’s atoms get so close that they generate a quantum mechanical force called electron degeneracy pressure. An object held up by this pressure is called a white dwarf.

In 1930, the Indian physicist Subrahmanyan Chandrasekhar showed that, given enough mass, a star’s gravity could overcome this electron degeneracy pressure, squeezing all its protons and electrons into neutrons. Though a neutron degeneracy pressure could then hold the weight up, forming a neutron star, the physicist Robert Oppenheimer found that an even more massive object could overcome this final outward pressure, allowing gravity to win and crushing everything down to a single point. Scientists slowly accepted that these things were real objects, not just weird mathematical solutions to the equations of General Relativity. In 1967, physicist John Wheeler used the term “black hole” to describe them in a public lecture, a name that has stuck ever since.

But as researchers studied black holes, they kept finding new ways in which they were bizarre. In 1974, Stephen Hawking made his most famous discovery: black holes can emit radiation. Down at the subatomic scale, fundamental particles are constantly winking in and out of existence. (It’s freaky to think that this is happening all around you all the time but it’s true.) Hawking realized that if a particle and its antiparticle appeared just at the edge of a black hole’s event horizon something odd would happen. Typically, these two particles would annihilate one another and release their energy back to the universe. But if one of the pair got sucked into the black hole, the other would get flung outward. The expelled particle would carry a bit of the black hole’s energy away, slowly sapping its strength. Given enough time, black holes would evaporate.

This introduced a dilemma: What exactly happens to that particle falling into the black hole? According to the laws of quantum mechanics, information about that particle cannot be destroyed. But once a particle has slipped beyond the event horizon, nothing about it, including its quantum mechanical information, can be recovered. Or at least, that’s what Hawking argued for more than 30 years, even going so far as to make a famous bet with physicist John Preskill in 1997 that the intense gravity of the black hole somehow overpowers the laws of quantum mechanics. But in 2004, Hawking had to concede that he was wrong. The work of other scientists, particularly string theorist Donald Marolf and physicist Juan Maldacena, showed incontrovertibly that there was no way to destroy quantum information and that it was actually leaking out of the black hole.

Despite the fact that everyone agreed on this conclusion, no one had any idea of exactly how it was happening. The result generated yet another intractable problem. The particle and antiparticle responsible for this whole mess were quantum mechanically entangled, which means that their properties are forever linked. Theoretical physicist Joseph Polchinski, working with Marolf and others, showed in 2012 that, in order for everything else known about black holes to be true, this quantum entanglement would have to be severed. Such a process is possible but violent. Disentangling the twins would generate a huge amount of energy right at the black hole’s event horizon. This energy would form a wall of blazing fire around the black hole, incinerating anything that came near.

Yes, we’re getting to the bit about Hawking’s recent proposal. Just hold on. Nothing about black holes is quick and easy.

The searing firewall solution has yet to be fully resolved in the physics community. Some believe it, some don’t. But even for those inclined toward the idea, it creates, you guessed it, another major problem. According to Einstein’s relativity, an astronaut falling into a black hole shouldn’t notice anything particularly different about the universe when she passes the event horizon. Getting incinerated certainly counts as “something peculiar going on here.” Scientists are faced with a choice: give up on Einstein and acknowledge that firewalls exist or give up on quantum mechanics and realize that information gets destroyed in a black hole.

A fanciful image of what the heart of a black hole might look like. Of course, new theories could change this view. Image: NASA/JPL-Caltech

In an attempt to thread the needle between the options, Hawking recently proposed to do away with something else: the event horizon. He instead suggests that physicists consider a much more nebulous term called the “apparent horizon” around a black hole. The apparent horizon works a lot like an event horizon, in that light beams inside of it can’t escape. They get stuck in a holding pattern, moving as fast as they can to stay in the same place, like the Red Queen in Alice in Wonderland. In this way, light and other information can get sort of “stored” in an apparent horizon but eventually find its way out.

The apparent horizon and the event horizon don’t entirely overlap. Because a black hole can grow (by sucking in extra mass), its event horizon may move outward past its apparent horizon. Or, by releasing Hawking radiation, a black hole can diminish, shrinking its event horizon inside of its apparent horizon. The entire concept is pretty strange, even to physicists, and seems to depend on the way that you decide to slice up space-time in your equations. In his latest paper, Hawking is saying that the apparent horizon is the only real thing, allowing scientists to ignore the event horizon and the firewall/Einstein problems arising there.

Using an apparent horizon actually solves one other problem, namely how information can escape from a black hole. Because the apparent horizon’s trap is only temporary, radiation can leave the black hole and carry information, albeit in a very scrambled and chaotic form.

So does that mean that black holes’ problems are done away with? Absolutely not! Firstly, it will take some time for the physics community to digest exactly what Hawking is suggesting. Already, some scientists are for it while others aren’t so sure. The one thing that can basically be guaranteed at this point is that physicists will eventually find some other mind-bending property of black holes that seems to contradict everything we thought before.